1. Field of the Invention
The present invention relates generally to a method of manufacturing complex structures from fiber-composite materials and more particularly to the construction of a fiber-composite turbine blade having an increased capacity to withstand centrifugally induced loads.
2. Description of the Prior Art
The key to extracting improved performance in advanced liquid propellant rocket engines lies in the development of improved turbopumps having components which withstand elevated operating temperatures. Substantial interest has thus arisen in the development of materials and construction techniques for turbine blades, vanes and other components which can withstand these elevated conditions.
Tungsten fiber reinforced superalloys (TFRS) are the first of a family of high temperature composites that offer potential for significantly raising the allowable operating temperature of turbine components and they generally comprise plies of spaced apart, parallel tungsten alloy fibers set within superalloy matrix material. TFRS composites are potentially useful as turbine blade materials because they have many desireable properties such as good stress-rupture and creep resistance, oxidation resistance, ductility and microstructural stability. TFRS composites are usually formed into monotape plies which are bonded together to form the desired component.
Although TFRS composites provide all these advantages, substantial problems do arise when it is desired to construct a component having integrally formed transverse members such as a cylinder with a rim, a vane with a shroud or a blade with a footing. Generally, the past practice for forming such structures with TFRS composites has been to form the members separately and then bonding the elements together along an interface. This construction however weakens the final structure against shears acting along the plane of the bond interface.
For instance, the practice for constructing TRFS turbine blades has generally comprised the steps of forming the aerofoil portion of the blade from a lamination of die cut plies of TRFS monotape. The footing of the blade is then formed by bonding separate pieces of material to the outer and inner sides of the root section of the blade. This scheme for attaching the footing, however, is a weak link in the overall structure because rotation of the turbine disk subjects the interface between the root of the aerofoil and the footing to a large and very concentrated shearing-load, particularly in the more advanced turbopump designs wherein shaft speeds can be 40,000 to 90,000 revolutions per minute. These speeds, along with the elevated temperatures and weight of the TRFS material, cause the risk of a shear-mode failure in this type of blade to be undesirably high.